Generally, in a FDM system a transmitter transmits data on different RF carrier frequencies. Typically each carrier frequency conveys an independent data stream, and a single carrier serves a particular user. In order to increase the data rate, two or more carriers can be grouped together such that the user is capable of receiving data simultaneously from different carriers. Such a scheme has been proposed for use in, for example, a 3xEV-DV system, where three adjacent 1xEV-DV carriers form the system, while preserving the 1xEV-DV compatibility for each carrier.
As currently specified, the cdma2000 MC system evenly distributes downlink traffic (traffic going from a base station (BS) to a mobile station (MS)) to all of the forward link carriers (to the three specified 1.25 MHz, actually 1.2288 MHz, carriers). As currently specified, the MS may be assigned one carrier or three carriers to receive data, depending on the required Quality of Service (QoS).
Although the 3xEV-DV system has three times the bandwidth of the 1xEV-DV system, it is expected that the diversity order will not increase proportionally. This is because the carrier frequencies are separated by 1.2288 MHz, which is a small amount relative to the coherence bandwidth (delta fc). That is, the 3xEV-DV system may exhibit the characteristics of a frequency non-selective channel, in some situations.
As employed herein, the diversity order is considered to be the number of uncorrelated signal paths that exist between a transmitter and a receiver. As employed herein, the coherence bandwidth delta fc is considered as the maximum frequency separation of two sinusoids that are affected in a similar way by the channel.
As was noted, when a MC-FDM system uses adjacent carriers, it is possible that the diversity order does not increase because the overall system still has a bandwidth less than the coherence bandwidth. In this case it can be noted that the receiver must track the channel realizations for each carrier, since due to hardware imperfections the bandpass filters for the sub-carriers are typically dissimilar (although the carriers may be correlated).
Considering the specific case of the 3xEV-DV system, the receiver may need to track three channels, one for each of the carriers employed. It can thus be appreciated that it would be desirable to increase the diversity order of a MC-FDM system that operates with frequency non-selective channels, and yet do so without increasing the number of channels realizations the receiver has to track (estimate).
A common solution for this type of problem is to use transmit antenna diversity. However, and if for example two antennas are used at the transmitter, the receiver is required to track and estimate twice as many channels as for the one transmit antenna case.